In a diesel engine, the fuel injection quantity is controlled by varying the time period of electric current supply to one or more fuel injectors so as to vary the fuel injection time period. Accurate fuel injection is difficult, however, because the relationship between current supply period and fuel injection quantity differs between different injectors owing to differences in the performance of individual injectors, change with aging and the like. This difficulty is particularly pronounced in the low-load, low-rpm region where the injection quantity per injection is small and in split fuel injection that divides the amount of fuel injected during each stroke into two or more injections.
Japanese Patent Unexamined Publication No. 11-287149, having counterpart U.S. Pat. No. 6,053,150 and EP publication No.947686, teaches a device for overcoming this problem that detects the minimum current supply period required for fuel injection by determining the start of pilot fuel injection from change in common rail pressure.
On the other hand, Japanese Patent Unexamined Publication No. 11-343911, having counterpart U.S. Pat. No. 6,244,241 and EP publication No. 947684, teaches a device that estimates injection quantity per injection based on the output of an O2 sensor for detecting air-fuel ratio and detects the relationship between current supply period and injection quantity from the estimated injection quantity and the width of the injection pulse (current supply period) output at the time of injection. Japanese Patent Unexamined Publication Nos. 2000-227036 and 2001-98991 also teach same device.
The device of Publication No. 11-287149 cannot accurately detect fuel injection in the small injection region because the common rail pressure is affected by dynamic factors and the like. Moreover, while this device can detect the lower limit value of the current supply period required for fuel injection, it cannot accurately detect whether a quantity of fuel enabling ignition was injected, i.e., cannot accurately detect the lower limit of the current supply period required for stable ignition.
The device of Publication No. 11-343911 utilizes an Q2 sensor for air-fuel ratio feedback control that is peculiar to a gasoline engine and cannot be directly applied to a diesel engine.
An object of the present invention is to provide a diesel engine control system and control method that enable accurate fuel injection even at times of small quantity injection such as pilot injection.
Another object of the present invention is to provide a diesel engine control system and control method capable of conducting accurate fuel injection unaffected by variations in injector performance caused by differences among individual injectors or change with aging.